if it were me, i would scour the junk yards looking for old semi-trucks or buses. bring a tape measure and find a radiator that is nearly the same size as the one you have now. It doesn't have to be an exact replacement, it just needs to be close. Most skoolies have lots of extra space under the hood to build some little brackets to hold the new radiator. Do you have a gas or diesel engine?? If it's diesel, you might be able to find a radiator off of a slightly smaller vehicle....perhaps a 1 ton pickup and still have good luck keeping the engine cool. I know my 6.6 liter ford diesel barely produces enough heat to keep itself in the "normal" temp once she slows down to idle. If you skoolie is gas, you'll be forced to do it right and go with a really big radiator made for a vehicle the size of a bus.
Of coarse, the "new" junk yard radiator might not be in much better shape than the one in your skoolie.
I have extraordinarily good luck with stuff from the junk yard. Almost everything in my skoolie was either free or cheap. I would have a hard time justifying 500 bucks for a radiator for my bus. I'd probaly bid on a "new" skoolie at one of the local school districts for $500 and keep it for spare parts before i'd spend that much on one engine part. On the other hand......you could just spend the money and do it right the first time!
Quis custodiet ipsos custodes (who will watch the watchmen?)